Although the adult mammalian myocardium exhibits a limited ability to undergo regenerative growth, the intrinsic renewal rate is insufficient to reverse pathophysiologic cardiomyocyte loss. The ability to reconstitute lost cardiac mass could thus be of considerable therapeutic value. One approach to accomplish this entails transplantation of donor cardiomyocytes or cardiomyogenic stem cells. This application focuses on cardiac-resident stem cells which express the Stem Cell Factor receptor c-kit. The proposed experiments are based on the recent observation that approximately 3% of the c-kit+ cells isolated from neonatal mouse hearts can give rise to well developed cardiomyocytes when co-cultured with fetal or neonatal cardiomyocytes. These cells appear to be either cardiomyogenic stem cells, or committed progenitor cells which require a heart-like environment to manifest a cardiac phenotype. Similar analyses using adult hearts suggested that the cardiomyogenic activity of c-kit+ cells is markedly limited or completely absent at later stages of development. The studies proposed in this competitive renewal application will establish the utility of cardiomyogenic c-kit+ cells (isolated from early post-natal hearts, from in vitro differentiating embryonic stem cells, and from adult hearts) for cell transplantation-based interventions aimed at repopulating the myocardium. Specific Aim 1 will characterize the developmental profile of cardiomyogenic c-kit+ cells, determine the degree to which the c-kit+ cells can be amplified while maintaining cardiomyogenic potential, and test the hypothesis that these cells are better suited than fetal cardiomyocytes to repopulate the adult myocardium following intra-cardiac transplantation. Specific Aim 2 will test the hypothesis that relaxation of epigenetic tags during prolonged culture will unmask cardiomyogenic potential in adult heart-derived c-kit+ cells. Ultimately these approaches might be useful to reconstitute myocardial mass following cardiac injury.